1. Field of the Invention
The present invention relates to a camera provided with an illuminating optical system.
2. Related Background Art
FIG. 14 is an elevation view of a camera provided with a conventional illuminating optical system.
The camera shown in FIG. 14 is provided with a camera body 1, on which there are provided, on the front side (object side) thereof, with a phototaking lens 2, an illuminating unit 4 and a view finder window 6 as illustrated in FIG. 14.
In order to avoid the red eye phenomenon (eyes of a person appearing red on the photograph), the illuminating unit 4 is positioned as far as possible from the phototaking lens 2. More specifically, when the camera body 1 is viewed from the front side, the illuminating unit 4 is positioned at the upper right or upper left corner of the camera body 1.
FIG. 15 shows the relationship between the range of illumination by the illuminating optical system and the range of phototaking by the phototaking lens, and FIG. 16 shows the structure of the illuminating optical system.
Referring to FIG. 15, a film plane 20 is positioned opposite to the object with respect to the phototaking lens 2, and the optical axis 3 of the phototaking lens 2 passes the center thereof and of the film plane 20. Consequently the phototaking range 17 of the phototaking lens 2 is defined laterally symmetrically with respect to the optical axis 3, by a line 7 connecting the center of the phototaking lens 2 with the right-hand end of the film plane 20 and by a line 8 connecting the center of the phototaking lens 2 with the left-hand end of the film plane 20. Stated differently, the phototaking range 17 of the phototaking lens 2 is defined by the paths 7, 8 of two outermost rays entering the phototaking lens 2.
The structure of the optical system of said illuminating unit 4 is shown, in a perspective view, in FIG. 16. The illuminating optical system is composed of a straight xenon tube (xenon flash lamp) 11, a Fresnel lens 12 and a reflector 13. There are also represented a longitudinal direction 14 of the xenon lamp 11, a central rotary axis 15 of the Fresnel lens 12, and a direction 16 perpendicular to the longitudinal direction 14 of the xenon tube 11 and to the central rotary axis 15 of the Fresnel lens 12.
In order that the phototaking range 17 of the phototaking lens 2 can be satisfactorily illuminated by the illuminating optical system, the position of the xenon lamp 11 is defined by a line 9 connecting the center of the Fresnel lens 12 with the right-hand end of the phototaking range 17 and a line 10 connecting the center of the Fresnel lens 12 with the left-hand end of the phototaking range 17. More specifically, the position of the xenon lamp 11 in the longitudinal direction 14 is defined by paths 9, 10 of the two outermost rays of the illuminating optical system.
The Fresnel lens 12 is positioned in the illuminating direction of the xenon tube 11, constituting the light emitting source, while the reflector 13 is positioned at the opposite direction, whereby the light emitted from the xenon lamp 11 is directed, either directly or after reflection by the reflector 13, by the Fresnel lens 12 to illuminate the phototaking range of the phototaking lens 2.
In such arrangement, if the illuminated range of the illuminating optical system is wider than the phototaking range 17 of the phototaking lens 2, a desired illumination intensity cannot be given to the phototaking range unless the amount of light emitted from the illuminating optical system is increased. On the other hand, if the illuminated range of the illuminating optical system is narrower than the phototaking range 17 of the phototaking lens 2, a desired illumination intensity cannot be obtained in a part of the phototaking range. For this reason, the illuminated range has been made as close as possible to the phototaking range.
The external shape of the Fresnel lens 12 has an aspect ratio matching that of the phototaking range of the phototaking lens 2. Since the xenon tube 11 is of straight form, the length Lx of the Fresnel lens 12 in the longitudinal direction 14 of the xenon tube is larger than the length Ly in the perpendicular direction 16. The reflector 13 is so formed as to surround the xenon tube 11.
FIGS. 17A and 17B are respectively a cross-sectional view and a plan view, showing the form of the Fresnel surface of a conventional Fresnel lens 12.
As shown in these drawings, the Fresnel lens 12 has a Fresnel surface 18 at the side of the light source (i.e. the xenon tube 11) and a flat surface 19 at the side of the illuminated range. The Fresnel surface 18 is constructed rotationally symmetrical about the optical axis 15. Consequently the refractive power of the Fresnel surface 18 is constant if the distance from the optical axis 15 of the Fresnel lens 12 is given.
In recent years, the camera bodies are designed in various forms, with increasing use of complex curved surfaces in which the curvature varies gradually. When the external form of the camera body is composed of such complex curved surface, it is desirable, for the aesthetic purpose, that the surface closest to the illuminated range (i.e. phototaking range) of the illuminating optical system, or the surface, closest to the illuminated range, of the Fresnel lens is also formed as a curved surface matching that of the camera body.
However, if the external form of the Fresnel lens is varied in order to give priority to the compactness or design of the camera body, it is not possible to obtain, by means of the Fresnel lens 12 only, the light distribution characteristics capable of satisfactorily illuminating the phototaking range of the phototaking lens, since the central rotary axis 15 of the Fresnel lens 12 is perpendicular to the Fresnel surface 18 as shown in FIG. 17A so that the light distribution characteristics is constant in every direction.
Also in the conventional camera employing a Fresnel lens with a rotationally symmetrical Fresnel surface as explained above, the refractive power of the Fresnel surface is constant in any direction, solely depending on the distance from the rotational center as explained in the foregoing. Consequently, if a complex curved surface is formed on the Fresnel lens at the side of the illuminated range, there will result a deviation in the light distribution in case of the conventional Fresnel lens of the rotationally symmetrical form, so that the phototaking range of the phototaking lens cannot be satisfactorily illuminated. Therefore the camera equipped with the conventional illuminating optical system has been limited in the freedom in designing the camera body.
An illuminating optical system in which the Fresnel lens is made eccentric on its surface at the object side (side of the illuminated range) is disclosed for example in the Japanese Utility Model Laid-open Application No. 2-138733. However, such illuminating optical system can correct the parallax to the optical axis of the phototaking lens but is unable to increase the freedom in designing the camera body, since the central axis of the Fresnel lens is still perpendicular to the Fresnel surface as in the conventional structure.
It is also conceivable to modify the light distribution characteristics by forming each annular part constituting the Fresnel surface into a rotationally asymmetrical shape, in which the Fresnel angle varies depending on the angle about the central axis, but such rotationally asymmetrical shape is extremely difficult to produce in practice.